In medical radiography, the image of affected tissue of a patient is formed by recording the pattern of X rays transmitted by the tissue in a photosensitive material which comprises a transparent support having thereon at least one light-sensitive silver halide emulsion layer (i.e., a silver halide photographic material). A transmission pattern of X rays can be recorded by using a silver halide photographic material alone. However, it is undesirable for the human body to be exposed to X rays in quantity, so that a combination of a silver halide photographic material with a radiation intensifying screen is generally used in practicing X-ray photography. The radiation intensifying screen comprises a support having a phosphor layer provided thereon, and the phosphor layer functions so as to convert the X rays absorbed thereby to visible rays to which a silver halide photographic material has high sensitivity. Therefore, the intensifying screen can markedly improve the sensitivity of an X-ray photograph taking system.
For the purpose of further heightening the sensitivity of an X-ray photograph taking system, there was developed the method of using a both-sided emulsion film, or a silver halide photographic material having silver halide light-sensitive emulsion layers on front and back sides of a support respectively, and practicing X-ray photography in a condition such that the film is inserted between two radiation intensifying screens (which may be simply called "intensifying screen"). In ordinary X-ray photography, the above-described photograph taking method is adopted at present. The development of this method originated in that sufficient X-ray absorption was not achieved by the use of only one intensifying screen. More specifically, even if the amount of a phosphor contained in one intensifying screen is increased, the converted visible rays are scattered and reflected inside the phosphor layer since the increased content of the phosphor results in thickening the phosphor layer. Accordingly, the visible rays emitted from the intensifying screen strike divergently on the surface of the photosensitive material disposed in contact with the intensifying screen. In addition, the visible rays generating in the depth of the phosphor layer are hard to get out of the phosphor layer. Thus, the amount of effective visible rays emitted from the intensifying screen cannot be increased even if the thickness of the phosphor layer is increased excessively. On the other hand, the X-ray photograph taking method using two intensifying screens which each contain a phosphor layer having a moderate thickness has an advantage in that the X-ray absorption as a whole can be increased and effectively converted visible rays can be taken out of the intensifying screens.
The research for finding out an X-ray photograph taking system excellent in balance between image quality and photographic speed has so far been carried out continuously. For instance, there has been prevailingly used the combination of a blue light-emitting intensifying screen having a layer containing calcium tungstate as a phosphor with a spectrally unsensitized silver halide photographic material (e.g., the combination of Hi-Screen Standard and RX, both being the products of Fuji Photo Film Co., Ltd.). In recent years, however, the combination of a green light-emitting intensifying screen having a layer containing the terbium-activated oxysulfide of a rare earth element as a phosphor with an orthochromatically sensitized silver halide photographic material (e.g., the combination of Grenex 4 with RXO, both being the products of Fuji Photo Film Co., Ltd.) has come to be used, and has effected improvements in both sensitivity and image quality.
However, a silver halide photographic material provided with photographic emulsion layers on both sides has a problem of tending to suffer deterioration in image quality due to crossover rays. The term "crossover rays" used herein refers to the visible rays which are emitted from each of the intensifying screens arranged on both sides of a photosensitive material, are transmitted by the support (usually having a thickness of 170-180 .mu.m or so) of the photosensitive material and further reach the light-sensitive layer disposed on the opposite side, thereby causing deterioration in image qualities (especially sharpness).
For the purpose of reducing the above-described crossover rays, various arts have so far been developed. For instance, U.S. Pat. Nos. 4,425,425 and 4,425,426 disclose the arts of using spectrally sensitized tabular-grain emulsions having high aspect ratios as light-sensitive silver halide photographic emulsions. According to those inventions, it is possible to reduce the crossover rays to 15-22%. Moreover, U.S. Pat. No. 4,803,150 discloses the art of disposing a layer of a microcrystalline dye capable of being decolored by development-processing between the support and the light-sensitive layer of a silver halide photographic material. Such an invention enables the crossover rays to be reduced to below 10%.
On the other hand, there have been made attempts to find out X-ray photograph taking systems excellent in balance between image quality and photographic speed by combining a silver halide photographic material having photographic emulsions on both sides thereof with radiation intensifying screens under specified conditions. For instance, JP-A-02-266344 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-02-297544 and U.S. Pat. No. 4,803,150 disclose the X-ray photographing systems designed so that the combination of an intensifying screen arranged on the X-ray irradiation side (front intensifying screen) with a light-sensitive layer (front sensitive layer) may be different in spectral characteristic (sensitivity) from the combination of an intensifying screen arranged on the opposite side (back intensifying screen) with a light-sensitive layer (back sensitive layer) and, what is more, the front combination and the back combination may have different contrasts. Further, experimental results of the combinations of the products of 3M Co., Ltd. concerning silver halide photographic materials and radiation intensifying screens are reported in Photographic Science and Engineering, Vol. 26, No. 1, p. 40 (1982). More specifically, the report states that the combination of Trimax 12 (trade name, a commercial intensifying screen of 3M Co.) with XUD (trade name, a commercial silver halide photographic material of 3M Co.) is almost equal in sensitivity and sharpness (MTF) to the combination of Trimax 4 (trade name, a commercial intensifying screen of 3M Co.) with XD (trade name, a commercial silver halide photographic material), but the former combination is higher in NEQ (ratio of noise to output signal) than the latter. Further, the report teaches that the above-described results can be inferred from the fact that XUD shows higher sharpness than XD, while Trimax 12 shows higher X-ray absorption than Trimax 4.
If attention is devoted only to the quality of X-ray images, it goes without saying that high quality images can be obtained by the combined use of a low-sensitivity silver halide photographic material and low-sensitivity radiation intensifying screens. In using a low-sensitivity combination as described above, however, it becomes indispensable to increase an amount of X rays to which human body is exposed (exposure amount). Consequently, such a combination is undesirable for practical use. In the case of a mass examination in particular, wherein most of the subjects are healthy persons, it is impossible to use that combination in practice because it is necessary to strictly avoid an increase in exposure amount.
As mentioned above, various studies have heretofore been made so as to develop X-ray photographic systems excellent in the balance between the image quality and the sensitivity in various kinds of radiographic systems. However, the conventional methods for forming X-ray images that have heretofore been developed for medical X-ray photography for obtaining X-ray images of bones and gastric areas of human bodies could not still be said to be X-ray photographic systems satisfying both the high image quality and the high sensitivity. This is because it is extremely important to clearly observe the fine structure of a bone so as to medically examine the bone by means of the X-ray image of the bone and it is also extremely important to clearly observe the structure of the gastric wall so as to medically examine the gastric area by means of the double-contrast X-ray image of the gastric area. However, the conventional methods for forming X-ray images for medical examinations were not satisfactory in view of these requirements.
In addition, X-ray photography for forming X-ray images of bones and gastric areas involves other difficulties. In forming X-ray images of bones by radiography for medical examinations, it is necessary that both the bones through which a small amount of X-ray penetrates and the soft tissues therearound through which a large amount of X-ray penetrates are photographed to have densities satisfactory for easy examination by medical examiners. For this, if a soft contrast photographic system is employed, the image formed will be examined with ease as a whole but the fine structure of the photographed bone is difficult to observe and examine. On the contrary, if a hard contrast photographic system is employed, the fine structure of the photographed bone will be clear but the soft tissues around the bone are defaced to dark in the photographed image so that they could not almost be observed or examined with an ordinary Schaukasten (film viewer). Medical examinations of gastric areas by double-contrast radiography have the same problems. It is difficult to observe and examine both the fine structure of the gastric wall to which a contrast medium of barium salts has been adhered and the inside of the upper area of the stomach filled with gas (gastric bubble) by means of one X-ray photographic image obtained by double-contrast radiography. This is because the amount of X-ray transmission noticeably varies in the different parts and areas of a stomach and therefore medical radiography for forming X-ray images of gastric areas needs a broad latitude, while the fine structures of gastric areas must be clearly observed and examined. It is extremely difficult to satisfy all the necessary requirements in medical radiography.